Musnad Ahmad 1153:413

Ibrahim ibn Abi al-Abbas narrated to us, Sharik narrated to us, from Ibrahim ibn al-Muhajir, from Mujahid, from al-Sa’ib, from Aisha, who attributed it to the Prophet (marfu’an), he said: "The prayer of the one sitting is half of the prayer of the one standing, provided he is not sitting cross-legged."